Background: Amlodipine decreases myocardial oxygen consumption (MVO2) via R+ enantiomer mediated endothelial nitric oxide (NO) release. We investigated the role of angiotensin (AT) 2 and 4 receptors in mediating this effect. Methods: We measured MVO2 in vitro using the Clark type oxygen electrode in isolated LV myocardial segments obtained from (i) wild-type and AT2 receptor knock-out (KD) mice (n=5), and (ii) explanted failing human hearts obtained at the time of heart transplantation (n=2). We studied the effect of increasing doses of R+ enantiomer (10-7M -105 M) on MVO2 with and without (i) NO synthase inhibitor, nitro-L-arginine methyl ester (LNAME, 10-3 M), (ii) AT2 receptor blocker, PD 123319, 10-6-10-5M, and (iii) specific AT4 receptor blocker, divalyl angiotensin 4,10-5M. Results: Wild-type mice: R+ caused a dose-dependent decrease in MVO2 (-24±8% at highest dose, p<O,05). This effect was inhibited by L-NAME (-17±7%) and 10-5M PD 123319 (2±7%). AT2 KO mice: R+ caused a dose-dependent decrease in MVO2 (25±3% at highest dose, p<0.05) and this was not blocked by 10-6M PD123319, selective AT2 receptor antagonist suggesting an alternate receptor/pathway. At higher concentration of 10-5M, PD123319 blocked the effect of R+ (-5±2%, p<0.01). At this dose, PD compound appears to be a nonselective AT receptor antagonist possibly acting on AT4 receptor, L-NAME caused a 33% reduction in the effect of R+. Human myocardium: R+ decreased MVO2 with a -20±1% decrease at highest dose. This effect was attenuated by both L-NAME and 10-5M PD123319. Also, specific AT4 receptor blocker caused a 50% attenuation of R+ effect. Conclusion: The R+ enantiomer of amlodipine decreases MVO2 by AT4 receptor mediated NO release in AT2 receptor knock-out mice. In failing human myocardium, this effect also appears to be mediated by AT4 receptors.